1. Field of the Invention
This invention relates a welding device for butt welding pipes and a method thereof.
2. Description of Related Art
When gas supply pipes are laid from a gas supply source in a gas yard to a point of use in a clean room, pipes are fixed in the gas yard, on walls of buildings, in pipe spaces in the buildings and under floor of the clean room, etc. Since the supply pipes are usually laid along the walls of rooms or buildings, a portion of the pipes are laid perpendicular or parallel to the other portion of the pipes. When the laying position of the pipes can be freely selected, for example, laying the pipes under the floor, most of the pipes are laid perpendicular or parallel to the partitions of the room. If a portion of the pipes is tilted towards the other portion of the pipes, there will be a loss of space and a difficult maintenance job, and furthermore, the appearance will be affected. In laying the supply pipes, it is better to consider a clearance of the pipes and a minimum clearance has to be reserved for welding at the pipe-laying site. Therefore, a common method is that pipe units linked by welding a plurality of pipes are made at a clean booth where the clearance can be controlled, and then the pipe units are welded according to a place to be laid.
A Japanese Laid Open Publication 2000-334564 discloses a method for wilding pipes. In Japanese Laid Open Publication 2000-334564, a groove sensor, having an emitting unit and a receiving unit for a laser beam, is used to irradiate the laser beam to grooves formed at the end of a pipe. A groove shape is detected according to the laser beam that projects the groove and reaches the receiving section. Thereafter, a welding torch is arranged according to the position of the detected groove so as to weld the pipe.
However, according to the method disclosed by Japanese Laid Open Publication 2000-334564, since the groove position is detected according to the laser beam that projects the groove and then reaches the receiving unit, the detection accuracy of the groove position is low and the welding position might be deviated from the pipe end.
FIGS. 3 to 5 show an exemplary welding device used to weld a pipe. As shown, the welding device comprises a head unit 1, an electrode unit 2, a fixing clamp 3 for holding pipes 4, 5 to be welded and a cover plate 9 formed on the fixing clamp 3. The head unit 1 comprises a base 11, a first and a second extension portions 12, 12 respectively extending upwards from the two ends of the base 11. The electrode unit 2 comprises an arc-shaped rotation member 21 and a welding electrode 22 formed on the rotation member 21. The fixing clamp 3 comprises a first clamp 31 and a second clamp 32. The first clamp 31 is used to hold one pipe 4 to be welded, and comprises a first and a second holders 33, 34 for sandwiching the pipe 4 both top and bottom. The second clamp 32 is used to hold the other pipe 5 to be welded, and comprises a first and a second holders 37, 38 for sandwiching the pipe 5 both top and bottom. The first holders 33, 37 are respectively formed in a plate shape having a pipe-holding recessions 33a, 37a that are conformed to outer circumferences of the pipes 4, 5. The second holders 34, 38 are respectively formed in a plate shape having a pipe-holding recessions 34a, 38a that are conformed to outer circumferences of the pipes 4, 5. The cover plate 9 comprises a cover body 92 and a transparent protection plate 93. The cover body 92 includes a position-identification window 91 for identifying positions of butt edge surfaces 4a, 5a of the pipes 4, 5 to be welded, and the transparent protection plate 93 its used to cover the window 91. The welding device can place the butt edge surfaces 4a, 5a of the pipes 4, 5 to be welded in a substantially sealed space S encompassed by the head unit 1, the fixing clamp 3 and the cover plate 9.
The method of using the above welding device to subject the pipes 4, 5 to be welded to a butt welding is described below. As shown in FIG. 5, a distance a1 between the outer-rim surface of the first clamp 31 and the welding electrode 22 is measured and a distance a2 between the outer-rim surface of the second clamp 32 and the welding electrode 22 is measured. A position indicator (a marking line) 51 is formed by using a pen, etc. at a position apart from the butt edge surface 4a of the pipe 4 to be welded by the distance a1. Also, another position indicator (a marking line) 51 is formed at a position apart from the butt edge surface 5a of the pipe 5 to be welded by the distance a2.
The positions of the position indicators 51 are respectively set to the outer-rim surface of the first and the second clamps 31, 32, and the pipes 4, 5 is arranged between the extension portions 12, 13 of the head unit 1 and then held by the clamps 31, 32. In this way, the pipes 4, 5 to be welded are positioned in a manner that the butt edge surfaces 4a, 5a are butted to each other.
For performing a good quality welding, it is important that positions (positions in the axial direction of the pipes 4, 5, and refer to an axial position, hereinafter) of the butt edge surfaces 4a, 5a of the pipes 4, 5 to be welded and the welding electrode 22 are consistent. In the above method, the positioning of the pipes 4, 5 to be welded in the axial direction is performed by matching the position of the position indicators 51 with the clamps 31, 32. When positioning the pipes 4, 5 to be welded, whether or not the axial positions of the butt edge surfaces 4a, 5a and the axial position of the welding electrode 22 are consistent with each other is identified through the window 91 by the unaided eyes. For performing a good welding, an axial position difference between the butt edge surfaces 4a, 5a and the welding electrode 22 is preferably equal to or less than a thickness half of the pipes 4, 5. In general, it is better that the axial position difference is equal to or less than 1 mm, and preferably 0.5 mm. After the positioning operation is performed, the electrode unit 2 is rotated along the circumferential direction of the pipes 4, 5 to be welded and the entire circumferences of the butt edge surfaces 4a, 5a are welded by using the welding electrode 22. In this way, the pipes 4, 5 to be welded are subjected to butt welding.
The position of the welding electrode in the welding device is varied due to the device itself. The welding electrode is not located at the central position of the two clamps, and might be shifted from any one of the clamps. Therefore, the position indicator (the marking line) has to be formed every time to meet the direction of the welding device.
However, when performing the welding at the pipe setting site, the position indicator 51 is not easily formed at the correct position and with a suitable width because the working environment, such as the working area, is not suitable for welding. If the position indicator 51 is not form at the correction position, the positioning accuracy of the pipes 4, 5 to be welded is low and thereby adversely affecting welding quality. In addition, if the position indicator is too thick, positioning the pipes 4, 5 to be welded might be incorrect. If the position indicator 51 is too thin, it is very hard to visually identify and positioning of the pipes 4, 5 to be welded becomes difficult.
Additionally, there is usually a gap of about 0.5 to 3 mm between the pipes and the electrode. Therefore, when identifying the axial position of the butt edge surfaces 4a, 5a through the window 91, the correct axial position has to be confirmed from a front direction of the pipes 4, 5 to be welded (a direction perpendicular to the axial direction). However, when the working area at the pipe setting site is limited, it is difficult to identify the axial position from the front direction. In addition, the identification might become difficult in a dark pipe setting site. Therefore, it is not easy to correctly and consistently position the butt edge surfaces 4a, 5a and the welding electrode 22.
If the axial directions of the butt edge surfaces 4a, 5a and the welding electrode 22 are shifted, the welding metal does not reach the inner surface of the pipes at the butt edge surfaces 4a, 5a, and a recession caused by the pipe gap is created on the inner surface at a position equivalent to the butt edge surfaces 4a, 5a. In this case, particles, etc. will be accumulated at the recession, and therefore, impurities such as particles will be mixed with a supply gas. In addition, when the supply gas is an erosive gas used in semiconductor manufacture, etc., the pipe will be eroded from the recession, and metal erosion product created by the erosion might be mixed with the supply gas.
Since the appearance of the welded portion may be still of a good quality even though the welding was performed in a worse condition due to the position deviation, a worse welded pipe is difficult to distinguish. In addition, when the groove shape is a V or U shape, the target position of the electrode for a normal distance between the electrode and the pipe is different from target position of the electrode when the axial direction of the pipe is shifted. Therefore, the position deviation can be determined by monitoring an arc voltage. However, in a case of an I-shaped butt welding, since the distance between the pipe and the electrode does not vary even though a position deviation occurs, it is difficult to determine whether the welding is of good quality by monitoring the arc voltage. Therefore, a method capable of performing a good welding is desired. In particular, for a case of supply pipes used for high purity gas, toxic gas (Arsenic, etc.), combustible gas (silane, etc.) in the semiconductor manufacture, a precise welding is desired in order to prevent gas leakage or impurity invasion from the pipe.